I think 16-20 plants is pretty ambitious with only one light, regardless of wattage. If you veg for any length of time at all, the plants will take up so much room that it will be difficult to get a high enough concentration of light on each and still cover all the plants. You've got plenty of room, but not enough light for the "footprint" that # of plants will cover. Consider going with 2 600's instead. That will allow coverage of a larger area, while also allowing you to get them closer to the canopy without burning the plants, especially if they are air-cooled, which I definitely recommend. Just put them end-to-end and cool them with 1 fan, probably an 8 inch if you can swing the $ for lights with 8 inch flanges. Good luck.